I recall first hearing ‘brilliant’ as a reference to exceptional intelligence in the 1970s.
In 1986, London band ‘Furniture’ released Brilliant Mind.
Are Brillo pads (soap-filled steel wool pot scourers) a thing in the US?

Yes, they are indeed Nick – I have some Brillo pads in the cupboard under my sink in San Diego. I don’t think of Brilliant as a Britishism per se, more the word being used more heavily in the UK than the US, much like “Awesome” but the other way around.